The present invention relates to the field of filtration and, in particular, to ultra-efficient filters, separators, and coalescers for separating entrained aerosols from a gas flow.
Certain gas streams, such as blow-by gases from piston compressors found in air recharge systems typically found in aircraft, carry substantial amounts of oils entrained therein, in the form of an aerosol. These oils are required to lubricate the piston compressor to ensure its long life operation. The majority of the oil droplets within the aerosol range in size from 0.1 microns to 5.0 microns. Coalescence methods using fibrous filters are generally used to remove this oil-based aerosol. Such methods rely on the following physical mechanism: (1) interposing a fibrous filtration means into a gas stream containing the aerosol so that the aerosol droplets are allowed to approach the fibers, (2) attachment of the droplets to the fibers, (3) coalescence of attached droplets on the fiber so as to create enlarged droplets, and (4) release of enlarged droplets to a collection area under the influence of gravity or centrifugal force when their weight exceeds a certain threshold.
Coalescent filters contain fibers, structured with various pore sizes, that are adherent to the aerosol. These filters are sometimes combined with a particle filter and a separator, such as an oil separator, to remove contaminant particles from the stream or to remove oils present in the gaseous stream for return to the sump of the piston compressor for reuse as lubrication. Additional filters that can be used include composite fiber-mesh filters and the like. Mesh filters contain fibers of, for example, polyester, polypropylene, nylon, Teflon, Nomex®, Viscose, or combinations of these materials. These fibers have a variety of pore sizes and are commercially available. Nomex® is a registered trademark of E. I. du Pont de Nemours and Company, Wilmington, Del. It is a polyamide in which all the amide groups are separated by meta-phenylene groups; that is, the amide groups are attached to the phenyl ring at the 1 and 3 positions.
Aircraft environments present a special set of problems for such oil coalescers. Cabin air is generally obtained from the blow-by air stream from the turbine engines propelling the aircraft. This air stream is extremely hot, has a high velocity, and contains an aerosol of oil and other contaminants produced by the turbine engine or by auxiliary compressor components. Removal of contaminants from such an air stream imposes unique requirements upon filtration and conditioning systems therefor, and in particular, upon oil coalescer devices.
U.S. Pat. No. 6,355,076, issued to Gieseke et al., discloses an oil separation and coalescing apparatus for removing entrained oils from an aerosol. It comprises a first coalescer filter with a non-woven media of fibers having a panel construction and a second coalescer filter with a pleated construction. It is designed for applications with diesel engines such as those typically found in trucks. Its temperature and gas velocity limitations are those typically found in trucks and not in aircraft environments, and more specifically, in aircraft piston compressors.
Oil coalescers, such as those made, for example, by Micro-Filtration, Inc., a subsidiary of Numatics, headquartered in Lapeer, Mich., are typically used to remove contaminants from gas streams. Such oil coalescers have been shown in testing to be unable to withstand the pressures resulting from high velocity gas streams, resulting in the disintegration of the coalescing element, i.e., filter; the particles of the disintegrated oil coalescer element are swept downstream of the oil coalescer to clog other system components and ultimately cause them to fail. They do not exhibit the structural integrity necessary to withstand the decompression rates and high temperatures present in the aircraft operational environment. Furthermore, the oil removal efficiency of these fiber-porous coalescers is only about 90% to 95%. It is desirable to attain an ultra-efficiency of 97% or higher for oil coalescers in an aircraft environment.
Oil coalescers currently used in aircraft air conditioning systems are typically bulky and have a moderate oil removal efficiency of 75%–85%. Such efficiency becomes more difficult to attain, as the coalescer becomes more compact. Because of the premium placed on space in an aircraft, it is desirable that the oil coalescer be light and compact, in order to augment operational efficiency of the aircraft and allow it to carry more equipment. Finally, a long service life of 10 years or more is desirable since it reduces the maintenance requirements for the aircraft piston compressor, making it cheaper to operate.
As can be seen, there is a need for an oil coalescer for use in an aircraft piston compressor for the removal of entrained aerosols from a high velocity gas, where the coalescing element is ultra-efficient (i.e. oil removal efficiency in excess of 97%) and vibration resistant. It is also desirable to provide an oil coalescer that is compact, has a long service life, and rugged enough to endure a continuous pressure presented by a high velocity gas stream and the decompression rates found in the aircraft's operational environment.